The present invention relates to a combined control method for a d-c-tie connecting two independent three-phase a-c networks.
In known control methods for a d-c-tie, the active power of the system is controlled and the reactive power is adapted to the reactive power demand of the respective a-c network with the aid of shunt reactors or capacitors, static compensators (for instance, thyristor-controlled reactors or capacitors) and/or synchronous condensors. For the controlled compensation devices for both a-c networks, a considerable amount of means is required for a necessary fine-step or stepless voltage adaptation to the active power to be transmitted. In addition, it is possible to switch the tap changer of the converter transformer in the wrong direction before the control arrangement can ascertain that compensating devices must be switched, so that the tap position must be switched back in the direction of its starting position after the respective compensation device has been switched by it independently; switching beyond its starting position may even become necessary. The control process with successive switching actions of the tap changer allows only a slow control; the required active and reactive power cannot be maintained during switching operations which take a relatively long time. In addition, the tap changer is stressed unnecessarily by the back and forth stepping operations.
In order not be extend the control time further, only the side of the d-c-tie connected to the weaker a-c network is frequently controlled in the sense of meeting the predetermined reactive power conditions of this a-c network, and the other side of the d-c-tie associated with the stronger a-c network is made to approach, as far as reactive power is concerned, the demand only via switchable shunt reactors or capacitors as compensating elements, so that in the second a-c network, the reactive power required there cannot be maintained exactly. This method further cannot recognize a control into undesirable boundary regions so that in the event of changes of the desired power or in the case of voltage changes in the a-c networks, no sufficient control reserve is provided. If in the case of two weak a-c networks an exact control is required also on the side of the second a-c network a further slowing-down of the control is resulted.